Increased network density and increased heterogeneity are among the key adaptations complicating the design and implementation of wireless communication networks. Network designers and operators must balance the necessity of having good coverage, at least in areas of high use, and having the right type of coverage, e.g., high-data rate coverage, against the enormous capital and operating expenditures needed to deploy and maintain the kind of equipment needed to ensure that those necessities are being met.
In one approach to increasing network density, rather than simply adding more “macro” or large-cell base stations, network operators are deploying smaller, low-power base stations, or allowing third-parties, such as individual homeowners or other subscribers, to deploy such base stations. These base stations characteristically provide radio coverage in much smaller areas, e.g., only within the confines of a typical residence or office. Consequently, these coverage areas are often referred to as “pico” or “femto” cells.
The base stations or access points, APs, that provide small-cell coverage may or may not use the same Radio Access Technology, RAT, in use in the macro-layer of the network, and varying degrees of integration are contemplated for APs with respect to the network at large. For example, the APs may or may not be part of overall coordinated interference reduction schemes that coordinate scheduling or other operational aspects of the network across and between cells.
Merely by way of example, a network operator may lease or sell small, low-power APs that individual subscribers install in their homes or workplaces. These APs may provide better baseline coverage or they may act as higher data-rate hotspots and, as such, they may have broadband connections back to the operator's network. In a particular approach, the APs couple to the operator's network through a controlling gateway. In such implementations, the AP has an air interface for connecting to devices, and has one or more network connections, often “wired” connections, back to the controlling gateway, which in turn has some type of “backhaul” connection to the operator's core network.
The gateway arrangement provides a number of advantages. For example, one gateway may support more than one AP. Consequently, at least some of the processing can be consolidated in the gateway. The centralization of certain Radio Access Network, RAN, processing functions is a topic of growing interest, and it is envisioned as a key aspect of future-generation network implementations.
Broadly, the idea here involves dividing the overall air interface operations and management processing between the actual radio access points providing the radio bearers and centralized processing nodes that provide relatively cheap pools of processing resources that can be leveraged for potentially large numbers of radio access points, also referred to generically as “base stations”. The lower-level functions, such as radio resource allocations and dynamic user scheduling are performed at the radio access nodes, which provide the actual radio link(s), while at least some of the higher-layer processing is moved to a central location.
This kind of disaggregation of the overall air interface processing protocols generally involves some “splitting” of the radio protocol stack between a radio access point and the centralized processing node. To better appreciate the split stack approach, consider the radio protocol stack used in Long Term Evolution or LTE. A wireless device and a network base station configured for operation in accordance with the LTE air interface each implements a version of the LTE protocol stack.
Protocol entities in the device-side stack mirror and communicate with corresponding peer entities in the network-side stack. The LTE stack includes a physical or PHY layer, as its bottom-most layer, a Medium Access Control, MAC, layer above the physical layer, a Radio Link Control, RLC, layer above the MAC layer, a Packet Data Convergence Protocol, PDCP, layer above the RLC layer, and a Radio Resource Control, RRC, layer above the PDCP layer. For more details regarding these layers and their functions, the interested reader may refer to the following Third Generation Partnership Project, 3GPP, Technical Specifications: TS 36.201 for a discussion of the physical layer, TS 36.321 for a discussion of the MAC layer, TS 36.322 for a discussion of the RLC layer, TS 36.323 for a discussion of the PDCP layer, and TS 36.331 for a discussion of the RRC layer.
In the context of the aforementioned gateway arrangement, a residential or other such radio access point implements a portion of the radio protocol stack, with the remaining portion of the stack implemented at the controlling gateway. This arrangement provides the twofold benefit of simplifying the radio access point and leveraging the gateway node for supporting more than one radio access point. However, the connections between the radio access points and the gateway nodes necessarily have limited bandwidth, and it is not always desirable to pass all traffic along to the gateway node.
For example, assume that two devices are both connected to the same radio access point and one device is sending traffic to the other. This traffic is “local” in the sense that the source and target devices are both connected to the radio access point. Ideally, the radio node would receive such traffic from the source device and send it directly to the target device. This type of idealized local loopback avoids the waste associated with passing local traffic up to the gateway node, only to have the gateway node send it back to the radio access point, for delivery to the target device.
However, the split-stack architecture does not readily accommodate the local loopback function for local traffic. For example, in LTE, the PDCP layer provides, among other things end-to-end ciphering for user traffic and control signaling going between a connected device and the LTE network. The PDCP entity in the device-side LTE stack provides one endpoint for the encrypted flows, while the PDCP entity in network-side LTE stack provides the other endpoint. Thus, with a split stack and with the PCDP ciphering functionality resident in the controlling gateway, the radio access point simply passes encrypted traffic and signaling between a connected device and the controlling gateway and has no knowledge of the security keys being used.
In a known solution to this problem, the gateway node provides the same ciphering keys to the radio access point, thus enabling it to perform the network-side ciphering operations. But these ciphering keys are extremely sensitive and their distribution presents security risks to the network operator and the user of the involved device. Thus, in such solutions, the radio access point must be a “trusted” node and there must be physical and logical security arrangements in place that make it feasible to distribute the ciphering keys to the radio access point. However, these trust requirements are cumbersome in at least some use cases, and significantly restrict the flexibility and range of choices available to subscribers and the network operator for adding radio access points to the network.